"Based on Justin's false statements at his news conference, and the fact that Canada is charging massive Tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the Communique", Trump wrote.
Heading into the leaders' summit in La Malbaie, Que., there were deep concerns the G7 alliance was fast becoming a G6 plus one because of a widening gulf between the US and the rest of the group in key areas such as climate and trade.
And in Ottawa on Monday, International Trade Minister Francois-Philippe Champagne urged lawmakers to speed ratification of the Trans-Pacific Partnership that might offset losses in U.S. trade by binding Canada closer to Asia, Australia and Latin America.
Ottawa has announced dollar-for-dollar retaliation on the USA imposition of steel and aluminum tariffs on Canada.
White House economic adviser Larry Kudlow accused Trudeau of betraying Trump with "polarizing" statements on trade policy that risked making the US leader look weak ahead of a historic summit with North Korean leader Kim Jong Un.
Navarro also said Trump attending the G7 in Canada was a "courtesy" to Trudeau and that the President had "bigger things on his plate" in Singapore.
Hours after Trump withdrew his support for the joint statement and attacked Trudeau, White House economic advisor Larry Kudlow and trade adviser Peter Navarro drove the message home on Sunday morning news shows in an extraordinary assault on a close USA ally and neighbor.
Lucey reported from Singapore and Gillies from Quebec City.
Trudeau's itinerary was marked "personal" and he was not in the House of Commons.
British Prime Minister Theresa May "is fully supportive" of Trudeau and his leadership, a senior United Kingdom government source said, while European Council President Donald Tusk tweeted: "There is a special place in heaven for @JustinTrudeau".
Oil prices drop as supplies increase
And while purchases have waned as shale production has grown, America is traditionally one of the biggest customers for Saudi oil. The oil supply deal is set to continue through the end of 2018, but plans for its continuation were unclear for next year.
He said the European Union "should pay much more for Military!"
"I'm very proud of how, not only of our government but also opposition parties and premiers, business communities and civil society have all piled in on the same side in trying to negotiate a very hard situation".
Europe will implement counter-measures against USA tariffs on steel and aluminum just like Canada, German Chancellor Angela Merkel said, voicing regret about Trump's decision to withdraw support for the communique.
Trudeau fires back on May 31 over the tariffs being applied on the premise Canada poses a national security threat to the U.S.
Enraged, our trading partners in North America, Europe and Japan say they will respond to our tariffs by levying their own on a wide range of goods and services produced in the United States.
In the wake of Trump and his aides' comments, many domestic and foreign leaders came to Trudeau's defense.
After he left the G7 summit in Quebec on Saturday, Trump posted a series of Tweets criticizing Trudeau for staying firm on the counter-tariffs imposed in reaction to US tariffs on Canadian steel and aluminum. Photo taken June 8, 2018.
Having left the G7 summit in Canada early, Mr Trump's announcement on Twitter that he was backing out of the joint communique torpedoed what appeared to be a fragile consensus on a trade dispute between Washington and its top allies. Now there is a question mark over that.
At the summit, Trump played a wild card, suggesting that rather than both sides boosting retaliatory tariffs - as he has just done on steel and aluminum - they could declare for entirely free trade in the G7 zone.
European Commission President Jean-Claude Juncker is set to debrief Commissioners about the G7 and underline the commitment made by the EU in the communiqué to "continue to stand up for an global rules-based multilateral system".